|By Sarah N. Lynch and Makini Brice

WASHINGTON (Reuters) - A former Russian intelligence officer who worked with U.S. President Donald Trump's former top campaign officials Paul Manafort and Rick Gates was communicating with Gates late in the 2016 presidential campaign, according to court records filed by Special Counsel Robert Mueller's office.

The connection between Gates and the former intelligence officer, identified only as "Person A" in the records filed late on Tuesday, is significant because criminal charges brought against Gates and Manafort relate only to their lobbying work for Ukraine prior to the 2016 U.S. presidential election and do not delve into their activities during the campaign.

“The fact that an official who had an important role in the Trump campaign alongside (Paul) Manafort was dealing with an individual who he knew was tied to Russian intelligence is a big deal, as is the fact that Mueller decided to put that card face up on the table at this time,” said a person familiar with Mueller's investigation who spoke on the condition of anonymity.

“He’s playing chess, and moving that piece now suggests that no matter what Trump is saying about no collusion, that part of the investigation is still very much alive.”

The Russian government has denied meddling in the 2016 election, and Trump has denied any collusion by his campaign.

Gates pleaded guilty last month to lying to the FBI and conspiring to defraud the United States, and he has agreed to cooperate with Mueller's investigation into Russian meddling in the 2016 election and whether there was any collusion with Moscow by Trump's campaign. An attorney for Gates did not immediately respond to a request for comment.

Manafort has pleaded not guilty to charges in two indictments filed by Mueller's office.

The charges range from bank fraud and filing false tax returns to conspiring to defraud the United States, conspiring to launder money and failing to register as a foreign agent when he lobbied for the pro-Russian government of former Ukrainian President Viktor Yanukovych.

Lawyers for Manafort are seeking to dismiss the charges, arguing they have nothing to do with Russian interference and fall outside the scope of what Mueller is supposed to investigate, among other things.

A spokesman for Manafort, who is prevented by a court-imposed gag order from talking to the media, declined to comment.

GUILTY PLEA

Gates' alleged communications with the former intelligence officer were revealed in a sentencing document for former Skadden Arps attorney Alex van der Zwaan. He pleaded guilty earlier this year to lying to the FBI about his interactions with Gates and the former intelligence officer and is to be sentenced on April 3.

In the document, prosecutors said Gates and van der Zwaan were in touch with the ex-Russian intelligence officer, who also worked for Manafort's lobbying firm in Ukraine, in September and October 2016. The presidential election was in November 2016.

They also said that when van der Zwaan was interviewed by the special counsel's office, he "admitted that he knew" about the Russian connection because Gates had told him about it.

The description of Person A in the court records appears to match that of Konstantin Kilimnik, a Ukrainian who worked for Manafort.

In the past, he has denied having ties to Russian intelligence services.

Reuters was not able to locate contact information for Kilimnik, and his whereabouts are unknown.

The revelation that Gates knew Kilimnik had been a Russian intelligence officer could prove to be the most significant turn in Mueller’s investigation to date, said a person familiar with the probe into whether there was collusion between the Trump campaign and the Russian government.

Kilimnik also acted as an intermediary between Manafort and Russian oligarch Oleg Deripaska, a close ally of Russian President Vladimir Putin.

In July 2016 Manafort, then chairman of Trump’s presidential campaign, emailed Kilimnik asking him to offer Deripaska “private briefings” about the campaign, according to the Washington Post.

Deripaska has since filed a civil lawsuit in New York state court against Gates and Manafort that accuses them of defrauding him out of an investment deal.

The lawsuit claims that Kilimnik graduated from the Military Institute of the Defense Ministry in Moscow. It also says that Kilimnik worked for 10 years in the Moscow office of the International Republican Institute, a U.S. government-funded nonprofit.